A COMPARATIVE-STUDY OF HOLE TRANSPORT IN VAPOR-DEPOSITED MOLECULAR GLASSES OF RAKIS(4-METHYLPHENYL)-(1,1'-BIPHENYL)-4,4'-DIAMINE AND '-BIS(3-METHYLPHENYL)-(1,1'-BIPHENYL)-4,4'-DIAMINE

Hole mobilities have been measured in vapor-deposited layers of N,N',N rakis(4-methylphenyl)-(1,1'-biphenyl)-4,4'-diamine (TTB) and '-bis(3- methylphenyl)-(1,1'-biphenyl)-4,4'-diamine (TPD). The results are desc ribed within the framework of a formalism based on disorder, due to Ba ssler and co-workers. The formalism is based on the assumption that ch arge transport occurs by hopping through a manifold of localized state s with superimposed energetic and positional disorder. The key paramet ers of the formalism are sigma, the energy width of the distribution o f hopping site energies, and Sigma, the degree of positional disorder. For ?TB, sigma = 0.078 eV and Sigma = 1.7. The values for TPD are 0.0 77 eV and 1.6. The width of the hopping site energies can be described by a model based on dipolar disorder. The degree of positional disord er is in agreement with results observed for a wide range of vapor-dep osited molecular glasses and is attributed to packing constraints, The results show that transport in both compounds can be described by sim ple disorder-controlled hopping without invoking polaronic contributio ns.